1. Field of the Invention
This invention relates to laboratory and borehole instruments making use of nuclear magnetic resonance (NMR) techniques for evaluating characteristics of hydrocarbon-bearing formations.
2. Description of Related Art
The introduction of pulsed nuclear magnetic resonance (NMR) logging tools in the early 1990s has provided the oil and gas industry with powerful new methods for evaluating petroleum reservoirs. The initial applications of pulsed NMR logging tools were aimed at providing important rock quality properties such as lithology-independent total porosity, free- and bound-fluid porosity, and permeability.
As is well known, the rate of decay of the NMR signal can be described, for example, by a distribution of decay times, which are called transverse relaxation times (T2). It is customary to fit the measured NMR signals to a sum of several decaying single-exponential signals, each with amplitude and associated decay time T2. The fitting procedure is achieved by a mathematical technique known as inversion.
The measurement of diffusion has become an important function of NMR well logging devices. NMR signals are attenuated by the molecular diffusion of oil, gas, and brine molecules through gradients in the static magnetic field. This effect is the physical mechanism that underlies all stand-alone NMR fluid characterization methods. Molecular diffusion is the random motion of molecules. The molecular diffusion constant of a molecule determines the mean square distance that the molecule will move per unit time. The diffusion of gas and water molecules can be described by a single molecular diffusion constant. Crude oils, on the other hand, have distributions of molecular diffusion constants that reflect the diversity of molecular sizes among the various components. Small, lightweight molecules like methane and ethane are relatively mobile in the gas phase and have molecular diffusion constants (D) that are typically about an order of magnitude greater than those of water molecules. In contrast, intermediate-to-high-viscosity crude oils have molecular diffusion constants that are much smaller than those of water. Contrasts in the molecular diffusion constants of formation fluids are exploited by using specially designed NMR measurements that are sensitive to diffusion. The NMR measurements carried out by the NMR well logging device can utilize fixed field gradient protocols. The NMR data are then analyzed to provide oil, gas, and brine saturations.
Bench-top NMR analysis is also commonly carried out in a petrophysical laboratory setting where rock core samples are subject to NMR measurements that are sensitive to diffusion. Such laboratory NMR measurements typically utilize a homogeneous static magnetic field profile together with a pulsed magnetic field gradient generated by application of direct current through gradient coils placed in close proximity to the sample volume. Typically, the protocols utilized by the bench-top NMR analysis in the petrophysical laboratory setting are different in many respects from those carried out by NMR well logging devices due to:                the more homogeneous nature of the static magnetic field of the bench-top NMR analysis as compared to the static magnetic field of the NMR well logging device;        the preferred use of pulsed field gradients as opposed to fixed field gradients; and        the preferred use of constant time/variable gradient strength diffusion encoding as opposed to variable time/constant gradient strength diffusion encoding.        
Because bench-top NMR analysis typically utilizes a different protocol than NMR well logging devices, it is difficult to directly compare the NMR data produced by an NMR well logging device and bench-top NMR analysis with regard to a given formation sample.